Cellular network-controlled short range communications between wireless communication devices and the integration of ad-hoc networking in cellular networks have been the topic of research since the late 90's. It has been found that short-range communication can take advantage of a cellular control layer in distributing content between mobile users relying on unlicensed spectrum resources in a peer-to-peer fashion. In 3GPP Release 12, device to device (D2D) communications, or proximity services (ProSe) communications, are mainly motivated by maintaining communications capabilities in public safety situations, although D2D communications can be used in other situations as well. According to the associated requirements, ProSe communication has to work in regions, where network coverage cannot be guaranteed. Therefore, ProSe communications are specified for in-coverage, partial coverage and out-of-coverage scenarios (see FIG. 1).
In LTE, D2D communications are made possible by transmitting and receiving control signaling and user data on the sidelink, SL (that complements the communication services using uplink (UL) and downlink (DL)). Specifically, the 3GPP has specified physical layer (PHY) channels, including the PHY sidelink shared channel (PSSCH) and the PHY sidelink broadcast channel (PSBCH) to transmit data to a specific peer device and to broadcast user data to a set of peer devices using the sidelink physical resources. Currently, the sidelink PHY layer resources are a subset of the cellular UL resources. In future (5G) networks, it can be expected that sidelinks will use both cellular UL and DL PHY resources.
In 3GPP Release 12, the PHY layer for D2D communications can be based on broadcast services, that is the D2D transmitter does not address user data packets to a specific receiver node, but any node in the proximity can receive and decode the transmitted packets. Consequently, the broadcast transmission does not consider the large or small scale channel conditions of any particular receiver node. Therefore, the spectral and energy efficiency of the broadcast transmission is typically low. Furthermore, some of the intended receivers may not be able to receive the intended data packets, or the bit error rate (BER) of the received data packets is high and the experienced quality of service (QoS) is low.
Therefore, there is a need for an improved method for D2D communications in cellular networks.